This invention relates generally to digital imaging, and more particularly to x-ray detector imaging.
Radiation imagers, such as digital x-ray imagers, typically include a scintillator coupled to a photosensor array. The radiation to be detected, x-rays for example, are absorbed by the scintillator material with the release of electrons which are converted to optical photons inside the scintillator that in-turn are detected by photodiodes which accumulate charge corresponding with the incident photons. The charge is read out by drive electronics to provide electrical signals corresponding to the radiation image. Commonly the imager has a reflective layer disposed over the scintillator reflecting optical photons from the top surface underneath the scintillator back towards the diode detector located underneath the scintillator. Typically, the reflective layer is incorporated in digital x-ray imagers to optimize the capture of optical photons and thus increase the conversion factor (CF). The CF is a measure of the detector""s ability to convert x-rays into electrons. Thus, CF increases because more photons are incident on the photodiode array.
One undesirable side effect of the reflective layer is that it can reduce the panel""s modulation transfer function (MTF). The MTF is a measure of the relative amount of x-ray modulation on a scale of zero to one at a given resolution (line pairs per mm). The reflective layer reduces the detector MTF by scattering some of the light into adjacent pixels and thus reducing the panel""s resolution. The presence of the reflective layer on a CsI scintillator, for example, can reduce the Mm by 10 to 20 percent as compared to a scintillator without a reflective layer. Furthermore, over time, moisture or chemicals released from reflective layer when heated (e.g. in the range about 75 to about 85 degrees Celsius), as may occur in shipping, can adversely affect scintillator performance and result in an MTF loss of another 10 to 20 percent with respect to an un-heated panel.
In cardiac imaging the beating heart is x-rayed in real time and thus many images are exposed in rapid succession. As a result, there is an increase in radiation exposure to the hospital staff and the patient. Therefore, a highly reflective layer is desirable to reduce the x-ray dose while maintaining a good image. The highly reflective layer increases the number of photons captured per incident x-ray and thus, by definition, the CF increases. In certain types of imaging, for example, rad (chest) and mammography, the need for high CF afforded by a reflective layer is not as critical as in cardiac imaging. In such imaging, it is desirable to have an imager with reduced optical cross-talk between pixels and thus the MTF is increased. Accordingly, there is a need in the art for an improved radiation imager that provides an MTF appropriate for x-ray imaging.
A radiation imager includes a photosensor array disposed on a substrate. The photosensor array is optically coupled to a scintillator. A cover plate is disposed over the photosensor array, and the cover plate further comprises a light absorbing layer interposed between the scintillator and the cover plate.